A submillimeter-millimeterwave study of the molecular gas in the nuclear regions of three nearby starburst galaxies.

Abstract

In this thesis we use multi-transitional millimeter/submillimeter-wave molecular spectroscopy of CO and CS to determine the state of the molecular gas in the central regions of three starbursts: M82, IC342, and M83. High angular resolution 60 and 100 μm IRAS images provide complementary information about the thermal dust emission in IC342 and M83. Our CO observations reveal the presence of a molecular ring and supernovae driven wind in M82. In IC342 and M83 there is evidence for molecular bars and central rotating cores. The CO and CS line ratio analyses suggest a multicomponenet medium with clouds externally heated by ultraviolet flux from young, massive stars. Excitation temperatures typically range from 20 to 40 K throughout the nuclear regions of the sample galaxies. In M82 the CO and CS optical depths are ∼ 1. Our analysis of ¹²CO indicates that this gas is optically thick toward the centers of IC342 and M83. The molecular gas mass in each galaxy is ∼ 5x10⁷ M(⊙). We derive an average cloud size between 0.1 and 1 pc in the nuclear region of M82 and M83. An average cloud size of 10 pc is found over a comparable region in IC342. From tidal arguments we find that the clouds must have densities greater than 100 to 1000 cm⁻³ to survive. If the clouds are virialized, then the expected individual cloud linewidths are 9, 40, 5 and 27 km/s for M82, IC342, M83 and the Milky Way, respectively. For the clouds to be pressure-bound, inter-cloud pressures > 10x the peak value in the Galactic Center are required. If the magnetic fields are frozen into the gas, an average field strength of 8.5 mG is needed to support the nuclear clouds in each galaxy from collapse. Enhanced IRAS images reveal bright, compact nuclear components in IC342 and M83. HII regions are seen along spiral arms in IC342 and a dusty bar is seen in M83. The similarity between radio continuum maps and the high resolution IRAS maps suggest that infrared emission arises from HII regions. Using an emissivity law of β ∼ 1.5, the derived dust temperatures in the nuclei of IC342 and M83 are essentially the same as the gas excitation temperatures. For this to occur, gas densities of > 10⁴ cm⁻³ are implied. We derive a star-formation efficiency, ∊, of 77, 60, 10 and 2% for M82, M83, IC342, and the Milky Way, respectively. We find evidence that the gas surface density toward the centers of these galaxies is α ∊. We estimate star-formation rates of 16, 6, 2.5, and .06 M(⊙)/yr for M82, M83, IC342 and the Milky Way. The gas depletion timescales are a few million years for M82 and M83 and a few times 10⁷ and 10⁸ years in IC342 and the Milky Way. We find a strong correlation between cloud diameter and star-formation efficiency, with smaller clouds found in galaxies with higher ∊. We conclude these smaller clouds are a by-product and not a causal factor of the starburst phenomenon.